Capillary applicator for semiconductor alloying apparatus



July 27, 1965 CAPILLARY APPLICATOR FOR SEMICONDUCTOR ALLOYING APPARATUS K. LEHOVEC Original Filed July 6, 1959 IOO lk\ I /ll II2 IIO 72 l//l IOO 37 x ::z x/

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KURT LEHOVEC HIS ATTORNEYS i United States Patent O 3,196,830 CAPILLARY APPLHCATOR FOR SEMICONDUCTOR ALLOYNG APPARATUS Kurt Lehovec, Wiliiarustown, Mase., assignor to Sprague Electric Eonpany, North Adams, Mass., a corporation of Massachsetts Original application July 6, 1959, Ser. No. 825,181, now Patent No. 3,097,976, dated July 16, 1963. Divided and this application May 18, 1962, Ser. No. 204,!)25 3 Claims. (CI. 118-401) This application is a division of Serial No. 82S,181 filed July 6, 1959, and issued as U.S. 3,097,976 on July 16, 1963.

This invention relates to apparatus for alloying metal to semiconductors, and more particularly relates to apparatus for producing alloy electrodes by liquid column alloying techniques whereby a small area alloy contact to a semiconductive crystal is obtained by bringing a droplet of molten metal or metal alloy from a large reservoir into contact with the semiconductive crystal.

There are two presently popular semiconductive devices in which the electrical properties are controlled by maintaining control over the lateral extent and the separation of closely spaced electrodes. Control over separation must include control over the depth of penetration of the electrode into the semiconductive crystal. One of these deVices is known as the electro-chemical transistor, and includes such sub-types as the surface barrier transistor, the microalloy transistor, and the microalloy diffused transistor, and is characterized by a semiconductive Crystal having a narrow web produced by jet etching and having emitter and collector electrodes on opposite sides of the narrow Web. The fabrication of the narrow web transistor is disclosed in detail in an article by Tiley and Williams in Proc. IRE 41, (12) 17064708 (1953).

The other of the two semiconductor Construction-s that requires careful control over the extent and penetration of electrodes is called the mesa semiconductive device, and is characterized by a flat-top elevated portion (mesa) having steeply sloping wal-ls rising above a surrounding substantially flat surface. Two cl osely spaced electrodes are plated onto the small area flat-topped portion of the mesa. A suitable method for preparing a mesa transistor having two electrodes on its elevated mesa is described in New Transistor Design- The Mesa' by C. H. Knowles in Electronic Industries, pp. 5540, August 1953.

Recently, a combination of the two aboVe-mentioned types of semiconducting devices has been devsed. This new device includes a small area mesa surrounded by a moat or trough that extends into the crystal body toward the opposed face that has been concavely etched by means that is typical of the electro-Chemical transistor. The trough extends toward the etched undersurface of the crystal -so that the space charge layer at the collector will reach the trough before teaching the emitter contact, thereby pinching-oif the mesa electrically from the base. This narrow webbed mesa semiconductive device is de scribed in detail in my Patent U.S. 3,087,099 issued April 23, 1963.

The problems involved in controlling the depth of penetration and the lateral extent of an alloyed contact are very exacting, inasmuch as the penetration can be no more than a few tenths of a mil and the lateral eXtent of an electrode can be no more than a few mils. One prior art attempt at meeting the problem includes the use of a mechanical jig that is positioned over a face of a semiconductive crystal so as to provide a form or guide that has a controlled area. A solid preform of the material to be alloyed to the crystal is then placed within this guide and a pressure-urged plunger means is inserted within the guide to exert pressure upon the preform while the &196330 Patented July 27, 1965 entire combination is brought to alloying temperature. In another prior art process, a droplet of alloy material is permitted to free-fall onto a semiconductor surface. The 'semiconductor surface and the drop must then be sub jected to conventiona-l alloying techniques in order to control the lateral extension of the drop. Neither of these two prior art processes provided a satisfactory solution to the problem, in that the first required time consuming and painstaking handling of tiny preforms, and the second failed to provide satisfactory control over the lateral eX tent of the drop.

My Patent U.S. 2,893,90l issued July '7, 1959 discloses and claims a process for producng an alloy junction in a semiconductor body by forcing a drop of molten metal or metal alloy from a reservoir through a restricted column into contact with a portion of a semiconductor body so as to dissolve the contacted portion of the body, and then recrystallizing the resulting liquid phase of droplet and semiconductor.

My Patent U.S. 3,097,976, of which the application is a division, discloses and claims a process improvement whereby a lead-wire is Secured to the small area alloy contact obtained according to the process of my Patent U.S. 2,893,901.

It is an object of this invention to provide apparatus for controlled production of small area alloyed Contacts for semiconductive devices.

It is another object of this invention to provide apparatus for controlled metering of liquid alloying material from a liquid column.

It is a further object of this invention to provide apparatus for positive controlled breaking of a column of liquid alloying material.

These and other objects of this invention will become apparent from the following description when read in conjunction with the accompanying drawing, in which:

FIGURE 1 is a `sectional view of one embodiment of apparatus according to this invention for controlled breaking of liquid alloying material from a liquid column;

FIGURE 2 is a sectional view of another embodiment of apparatus according to thi-s invention;

FIGURE 3 is a sectional View of a further embodiment of apparatus according to this invention;

FIGURE 4 is a sectional view of another embodiment of apparatus according to this invention; and,

FIGURE 4u is a sectional view of the apparatus of FIGURE 4 showing lateral displacement as a means for attaining the objects of this invention.

In general, the objects of this invention are obtained by apparatus for controlling the amount of liquid alloying material that is utilized in the process disclosed and claimed in U.S. 3,097,976. Briefly, that process comprises Contacting a semiconductive wafer with a molten metal alloy column at the mouth of a capillary and then removing the wafer with an attached droplet of alloy to permit recrystallization and solidification of the liquid phase comprised by the molten droplet and the contacted portion of the wafer without requiring contact with the rest of the alloy column. The apparatus of this invention provides control over the amount of alloying material removed from the capillary by providing means for positive controlled breaking of the liquid column.

A specific example of the process disclosed and claimed in my Patent U.S. 3,097,976 as practiced on the apparatus of this invention includes the preparation of an n-type germanium wafer with surfaces in the crystallographie (111) direction and containing antimony as an impurity to provide a resistivity of one ohm centimeter. The crystal is lapped to a thickness of five mils and then etched in a hydrofiuoric-nitric-acetic mxture to a thickness of three mils. The wafer is picked up by a suction tube and placed over the polshed plain surface of a glass capillary of mils internal diameter (approx. 100 mils outside diameter) contained in an inert atmosphere such as argon. The glass capillary is connected to a glass reservoir containing moltcn indium. The liquid indium is forced against the surface of the germanium wafer which has closed the orifice of the capillary by exerting pressure by means of a mechanical plunger or an inert gas pressure. A gas pressure of 40 centimeters of mercury has been found satisfactory for this purpose. The liquid indium is kept at a temperature of 250 C. and the germanium body is kept at a temperature of 200 C. Contact of the liquid indium to the germanium is maintained for thiry seconds and then the indium is withdrawn in the capillary by decreasing the pressure. This leaves a droplet of indium on the germanium surface, which solidifies upon removing the germanium from the orifice of the capillary.

An interesting embodiment of my invention involves drawing an indium wire in contact with an indium alloy contact to a germanium wafer. In order to accomplish the wire drawing, the pick-up suction tube described in the preceding paragraph is provided with tiny holes which permit blowing cool argon gas through the suction tube after the germanium is in contact with the liquid indium. By proper balancing and manipulation of the heating and cooling rates, a column of indium in the capillary ad jacent to the germanium can be solidified and the germanium together with this column can be removed from the capillary by utilzng suction in the suction tube as a pulling agent.

Various additional process embodiments are practical with the apparatus shown and described. For example, a p-n-p structure may be obtained by starting with a ptype germanium wafer and alloying thereto a dot of an alloy containing 97% indium and 3% arsenic. Inasmuch as the indium does not ditTuse as rapidly in germarium as does arsenic, the n-type zone is formed adjacent the p surface of the germanium wafer and is then overlaid with a p surface.

Another process embodiment of my invention involves the use of an alloy containing a doping agent for the Crystal, as well as a carrier chosen for its phase diagram and for its physical properties, for example expansion, 'softness Typical alloys for p-junctions to n-type germanium include indium, indium-gallium, and indiumaluminum; alloys for n-junctions to p-type germanium include lead-arsenic and lead antimony. These same alloys are also suitable for use on silicon crystals.

In order to produce well defined recrystallization of the liquid phase consisting of the alloying material and the dissolved portion of the semiconductor body, it has been found to be advantageous to utilize the well-known Peltier effect. A discussion of Peltier heating and cooling resulting from passing a direct current between a liquid and a solid is found in an article entitled Some Aspects of Peltier Heating at Liquid-Solid Intertaces in Germanium" by W. G. Pfann, K. E. Benson, and J. W. Wernick in Journal of Electronics (Eng.) (Ist series) 2, 597-608 (1956-1957). In the specific example recited above, wherein the molten material is indium and the semiconductor body is n-type gcrmanium, the piston 14 is made of carbon to permit the connection of the positive side of a D.C. supply to the indium while the germanium is made negative, whereby passage of current will produce a cooling effect at the interface.

In work done toward improving the reproducibilty of devices produced according to this invention, it was discovered that breaking the column of liquid alloying material from the resolidified contact on the semiconductor body sometimes resulted in pulling varying amounts of material from the contact to the body. This meant that even though withdrawing the pressure producing means resulted in a break-oli of the liquid column at some point within the capillary, the break-oli did not always occur at the desired position The nechanism involved in a faulty break-oti of the liquid column is believed to result from the tight seal that exists between the liquid column and the inside walls of the capillary. When the pressure producing means is released or withdrawn, the column will break otT at some point, thereby leaving an empty space or vacuum between the column and the material that has adhered or alloyed to the semiconductor body. This vacuum coacts with the atmospheric pressure outside the capillary to provide a suction. This sudden suction can pull down some of the adhered alloying material to such an extent that even when successive break-offs occur at one point, different amounts of alloying material may be drawn down, thereby lessening the probability of reproducibilty in the process.

Various techniques and equipment have been developed to ensure reproducibilty of the break-off point. These developments are illustrated in FIGURES 1 through 4a, which show a plurality of means for effecting break-oli of the liquid column at the exact point desired for optimum results. The constructions shown in FIGURES l, 2, and 3 have the common feature that may be defined generally as break-off means having a small radius of curvature whereby a high surface tension may be obtained. These break-oti means are so constructed as not to be wet by the liquid alloying material.

In FIGURE 1 the break-off means comprises a constriction 78 near the top of the capillary 72. This constriction 78 may involve the simple reduction of the internal diameter of the capillary 72 atthe desired point, or may comprse a plurality of projections that extend into the capillary. In FIGURE 2 the means to break the column i shown as a hemispherical orifice 88 at the top of the capillary 82. While the FIGURE 2 illustration is of a true hemispherical orifice, it should be understood that other shapes are within the concept of this feature of the invention.

The FIGURE 3 means to break the column of liquid alloying material 96 at the desired point is shown as a side vent or capillary 98 of smaller diameter than the main capillary 92. This side vent 98 passes through the wall of the main capillary tube 92 so as to permit the incrt atmosphere surrounding the apparatus to enter on the column 96, thereby avoiding the vacuum situation described above. This side vent 98 also provides the desired geometry of having a small radius of curvature to assist in locating the break-off at a gven point in the main capillary 92. It has been discovered that this side vent 98 also acts as a pressure gage which reveals the amount of pressure being exerted on the liquid column 96 by revealing the amount of alloying material that has been forced into the side vent. In this regard, it should be noted that the surface tension of the liquid column 96 is such that under ordinary pressure almost no material will enter the side vent 98. This pressure gage aspect of the side vent 98 is important when alloying in a blind cavity such as is produced in the electrochemical transistors shown in FIGURE 6 of U.S. 3,097,976. The side vent 98 also permits the use of a wire or slide-rod to mechanically break into the liquid column 96.

FIGURES 4 and 4a show two positions of another means to remove a controlled amount of liquid alloying material from a liquid column 106 that is in contact with a semiconducting body 100. This structure includes a diaphragm or carrying member 110 which supports and positions a semiconductor body 100 relative to a channel 112 in the diaphragm 110. When the diaphragm and semiconductor body 100 are positioned as shown in FIGURE 4, with the channel 112 in axal alignment with the liquid column 106 within the capillary tube 102, it is possible to exert pressure on the liquid column to force the alloying material through the channel into contact with the semiconducting body and thereby effect an alloy junction as described in the preceding examples of this invention. In this regard, the diaphragm 110 serves all of the functions previously served by the capillary tube, eg. a form to limit the lateral extent of the contact. The surfaces of the diaphragm 110 and the orifice of the capillary 102 may be so constructed and arranged as to provide a slide-fit whereby no alloying material can escape through the joint, and Whereby the column 106 of alloying material may be cut by lateral displacement of the diaphragm and the capillary. FIGURE 4a shows the diaphragm 110 and the capillary 102 after relative movement has taken place so as to break the column 106 cleanly at the desired point without pullng any of the material from the channel of the diaphragm 110, thereby providing a high degree of reproducibility of alloy Contacts.

Since the time during which the liquid alloy is kept in contact wtih the semiconductor body is quite short, it is frequently desirable to assure uniform Wettng of the semiconductor body by plating the surface of the semiconductor with a metal film, e.g. indium, which then dssolves during the liquid alloying process.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments herein described except as defined in the appended clams.

What is claimed is:

l. In an apparatus for producing an alloy junction in a semiconductor body, the improvement comprising means to meter a controlled amount of liquid alloying material from a liquid column that is in contact with a semiconductor body, said means including a capillary through which said liquid column passes to said semiconductor body, break-oti means having a small radius of curvature in said capillary whe'eby a high surface tension is obtained to promote breaking of the liquid column within said capillary.

2. Apparatus as defined in claim 1 wherein said breako means includes a side vent of smaller diameter than said capillary and which passes through the wall of said capillary.

3. In an apparatus for producing an alloy junction in a semiconductor body, the improvement comprising means to meter a controlled amount of liquid alloying material from a liquid column that is in contact with a semiconductor body, said means including a capillary through which said liquid column passes to said semiconductor body, a diaphragm between said capillary and said semiconductor body, a channel in said diaphragm to provide access to said semiconductor body, said diaphragm movable from a position with said channel aligned with said capillary whereby the liquid alloying material may be purnped from said capillary through said channel to alloy with said semiconductor body to a position of lateral displacement between said capillary and said channel whereby the liquid alloying material in said channel is sheared from the liquid alloying material in said capillary.

References Cited by the Examiner UNITED STATES PATENTS 2,849,34l 8/58 Jenny 148-179 2,893,901 7/59 Lehovec 148-185 X 3,097,976 7/63 Lehovec 148--179 RICHARD D. NEVIUS, Pr'mary Exam'ner.

JOSEPH B. SPENCER, Examiner. 

1. IN AN APPARATUS FOR PRODUCING AN ALLOY JUNCTION IN A SEMICONDUCTOR BODY, THE IMPROVEMENT COMPRISING MEANS TO METER A CONTROLLED AMOUNT OF LIQUID ALLOYING MATERIAL FROM A LIQUID COLUMN THAT IS IN CONTACT WITH A SEMICONDUCTOR BODY, SAID MEANS INCLUDING A CAPILLARY THROUGH WHICH SAID LIQUID COLUMN PASSES TO SAID SEMICONDUCTOR BODY, BREAK-OFF MEANS HAVING A SMALL RADIUS OF CURVATURE IN SAID CAPILLARY WHEREBY A HIGH SURFACE TENSION IS OBTAINED TO PROMOTE BREAKING OF THE LIQUID COLUMN WITHIN SAID CAPILARY. 